The present invention relates to a matching circuit of a microwave device and, more particularly, to a circuit having improved microwave characteristics.
As is conventionally known, a semiconductor amplifying element used in a microwave amplifier has a gain having a frequency characteristic of -6 dB/oct, and its input and output impedances are very high. The cut-off frequency characteristic of a semiconductor amplifying element has been improved and the wide frequency band has been widened in the course of development of semiconductor techniques. Therefore, to obtain a new microwave amplifier, impedance matching must be performed between the signal source and the microwave amplifier, and between the microwave amplifier and the circuit of the next stage, in order to reduce unnecessary reflection of microwave power and to obtain optimum microwave characteristics.
Impedance matching methods can be roughly divided into two categories.
According to the first method, a lossless network, using lossless elements such as a capacitor, an inductor, or a line and a stub formed on a dielectric substrate, is used.
According to the second method, a loss-including network, having a combination of a resistor and a lossless element, is used.
The second method is not used in a general application, except for special cases, since the gain of the amplifier is decreased due to loss. Consequently, the first method is used more often. A lossless network uses elements such as a capacitor and an inductor for frequencies up to the UHF band. However, as the frequency is increased, smaller values of capacitance and inductance are required with higher precision, and it is sometimes difficult to obtain the required characteristics, using elements such as a capacitor and an inductor. For this reason, in a circuit processing microwave signals, a matching circuit utilizing a microstrip line formed on a dielectric is employed.
More specifically, an impedance at an input/output terminal of a microwave amplifier generally has a value unrelated to frequency, i.e., characteristic impedance Zo in the frequency band used, e.g., 50 .OMEGA.. Since the input/output impedance of the microwave amplifying element (FET element) is high, it must be matched with characteristic impedance Zo. A matching circuit for this purpose consists of a transmission line and an open or short line. The transmission line has a specific line length and a self characteristic impedance, and can change the impedance and the phase with respect to a load impedance. The transmission line and the open or short line can change the admittance.
In the conventional microwave amplifier, when impedance matching is performed in the frequency band used, power is not reflected and the gain is increased. Since the reflected power is increased outside the frequency band used, the gain is decreased.
On the other hand, however, since the input- and output-matching circuits approach a matched state not only in the frequency band used (4-GHz band) but also at a specific frequency (7-GHz band) higher than this, the gain of the microwave amplifier is increased. In other words, in some cases the gain is great at frequencies higher than the 7-GHz band outside the 4-GHz band.
Stability factor K and the stability of a microwave amplifier have the following relationship. When stability factor K is 1 or more, the microwave amplifier is stable, regardless of its input/output load impedance. Otherwise, the microwave amplifier becomes unstable or oscillates, depending on the impedance of the input/output load.
With the conventional microwave amplifier described above, stability factor K becomes less than 1 in a 12-GHz band, and the microwave amplifier oscillates or operates unstably, depending on the input/output load impedance due to the influence of a reflected component. For example, assume that another circuit is connected to this microwave amplifier. When the power source is turned on, a noise reflection phenomenon occurs thereupon, and self oscillation can occur in the 12-GHz band. Therefore, when the conventional microwave amplifier using a lossless matching circuit is designed, not only the frequency band used, but also the maximum oscillation frequency and the load condition of its semiconductor amplifying element must be considered.